ABSTRACT
This chapter describes some thyroidological aspects concisely, aiming to identify less well-investigated areas of piscine thyroidology. It focuses on the teleost hypothalamus-pituitary gland-thyroid axis, the regulation of which allows bidirectional communication with the teleost stress axis. The chapter discusses the presence of heterotopic thyroid follicles in osmoregulatory organs. Early experiments by the thyroidology pioneer J. Friedrich Gudernatsch clearly indicated the determining role of the thyroid gland in metamorphosis. In contrast to the compact thyroid gland found in higher vertebrates, in virtually all teleosts and adult Cyclostomata the thyroid gland consists of rather diffusely scattered follicles in the region ventral to the pharynx, along the ventral aorta and where the branchial arteries branch off. Hypothalamic actions of thyroid hormones could well be involved in the synergistic or additive effects of thyroid hormones and cortisol in teleost osmoregulation. Treatment of animals with the thyrostatic thiourea apparently reduces their osmoregulatory capacity.
